Context Navigation

Changes between Version 4 and Version 5 of Ticket #147, comment 26

v4	v5
14	14	* re-doing the whole fragment program init everytime is wasteful, we should be able to load it up once and {{{glEnable}}}, {{{glDisable}}} it when needed by {{{paint_rgb24}}} (see example code attached which almost works)
15	15	* occasional {{{destroy_program_variants_cb}}} warning (see r2327) - maybe we do need to call {{{remove_shader}}} on the cleanup path
	16	* win32 crash: #229